1. Field of the Invention
This invention relates to the field of bandgap voltage reference cells, and particularly to bandgap reference cells having a high transconductance.
2. Description of the Related Art
A basic bandgap voltage reference cell is shown in FIG. 1. Two bipolar transistors Q.sub.a and Q.sub.b are driven by the output of an operational amplifier 14, with their collectors connected to the op amp's non-inverting and inverting inputs, respectively, and to a supply voltage V+ through respective resistors 16 and 18. A resistor R.sub.a is connected between the transistors' respective emitters, and a "tail" resistor R.sub.b is connected between the emitter of Q.sub.b and circuit common.
Q.sub.a is fabricated with an emitter area larger than that of Q.sub.b (by a ratio of 8-to-1 in FIG. 1). The op amp adjusts the transistors' base voltage until the voltages at its inverting and non-inverting inputs are equal. This occurs when the two collector currents match, which in this example happens when the emitter current densities are in the ratio of 8-to-1. This arrangement produces a voltage across R.sub.b that is proportional-to-absolute temperature (PTAT), which can be used to compensate the complementary-to-absolute-voltage (CTAT) characteristic of the base-emitter voltage of Q.sub.b. Setting OUT equal to the bandgap voltage of silicon provides the proper compensation, and thereby produces a temperature invariant output voltage.
The transconductance g.sub.m of the circuit of FIG. 1 is defined as the change in the difference in the transistors' collector currents divided by the change in their base-emitter voltage. Because the difference in collector currents cannot exceed the change in current through R.sub.b, the transconductance is capped at 1/R.sub.b, but because a perturbation causes both collector currents to change in the same direction, the maximum attainable g.sub.m is actually less than 1/R.sub.b. This bandgap reference cell and its characteristics are discussed in detail in A. Paul Brokaw's "A Simple Three-Terminal IC Bandgap Reference", IEEE Journal of Solid-State Circuits, Vol. SC-9, No. 6(1974).
Another bandgap reference cell is shown in FIG. 2, made from two transistors pairs connected in a "crossed-quad" configuration. A first pair of transistors Q.sub.c and Q.sub.d are connected in series with a second pair of transistors Q.sub.e and Q.sub.f, respectively, with the bases of Q.sub.e and Q.sub.f connected to the collectors of Q.sub.f and Q.sub.e, respectively. Transistors Q.sub.c and Q.sub.d have unequal emitter areas, as do transistors Q.sub.e and Q.sub.f. A resistor R.sub.c is connected between the emitters of Q.sub.e and Q.sub.f, and a tail resistor R.sub.d is connected between the emitter of Q.sub.f and circuit common. The collectors of Q.sub.c and Q.sub.d are connected to the inputs of an amplifier 20. The amplifier's output drives a pass transistor Q.sub.f to produce a regulated output OUT, which is fed back to Q.sub.c 's and Q.sub.d 's common bases. A PTAT voltage appears at the junction between R.sub.c and R.sub.d ; when the resistors are properly chosen, the PTAT voltage compensates for the base-emitter voltages of Q.sub.f and Q.sub.d to produce a temperature invariant voltage equal to twice the bandgap voltage at OUT. Achieving an output voltage greater that is a non-integer multiple of the bandgap voltage is typically provided by adding a voltage divider 22 between OUT and the common base connection, as shown in FIG. 2. The divider imposes a voltage drop between the output and the common base connection, but assuming that amplifier 20 has sufficient gain, it will continue to balance the collector currents and the output will be stabilized at a higher voltage.
The transconductance of the circuit of FIG. 2 is somewhat better than that of FIG. 1. When the cell is at equilibrium (i.e., when the collector currents are balanced), a PTAT current flows in R.sub.c which is determined solely by the emitter area ratios and the value of R.sub.c ; i.e., essentially independent of the current on the right side of the crossed-quad. With the left side current fixed, when the cell's output is disturbed, nearly all of the resulting change in current goes through the right side of the cell (Q.sub.d and Q.sub.f), with the current through the left side (Q.sub.c and Q.sub.e) essentially unchanged. Thus, all of the change in current goes through R.sub.d, and the cell's transconductance closely approaches 1/R.sub.d.
Because of the relatively low transconductance of the bandgap cells in FIGS. 1 and 2, the voltage applied to the common bases (of Q.sub.a and Q.sub.b in FIG. 1; Q.sub.c and Q.sub.d in FIG. 2) must depart substantially from the voltage which balances the currents if a large difference in collector currents is needed. This is usually accommodated by connecting a high gain amplifier across the collectors, to provide a differential-to-single ended conversion as well as the voltage gain necessary to return to equilibrium; this function is represented by amplifier 20 FIG. 2.
Disadvantages are found in the circuits of FIGS. 1 and 2, particularly when low power consumption is important, as with a battery-powered regulator. The power consumed by amplifier 20 will hasten the discharge of a battery used to provide the circuit's supply voltage, as will the energy lost in resistive divider 22. Use of a resistive divider 22 is also troublesome if the regulator is employed, for example, as a battery charger, with a battery to be charged connected to OUT. When the regulator is inactive or unable to provide the necessary charging current, the presence of a divider actually provides a discharge path for the battery, shortening its life.